1. Field of Invention
This invention relates to the detection of gases and vapors by microsensors. More specifically, this invention relates to microsensors having a film of a dithiolene transition metal complex which changes detectable physical properties upon exposure to particular gases and vapors.
2. Description of the Prior Art
A chemical microsensor for detecting gases or vapors is composed of a microelectronic device and a chemically sensitive film coated onto the device. The chemical coating determines the sensitivity and selectivity of the sensor. The film undergoes a change in a physical property upon exposure to a vapor or gas to which it is sensitive, and the microelectronic device detects this change. One type of chemical microsensor is the chemiresistor. One type of compound used to make films for a chemiresistor is phthalocyanine compounds.
A chemiresistor exhibits the electrical characteristics of a resistor which has its conductance changed by the presence or absence of some chemical species. Variations in the current flow signal the presence of specific vapors or classes of vapors.
U.S. Pat. No. 4,350,660 discloses that semiconductors have characteristic electronic conductivities which are strongly affected by ambient chemical vapors. U.S. Pat. No. 4,636,767 discusses investigations made on heated metal oxide semiconductors, such as tin oxide and zinc oxide for use in chemiresistors. These semiconductors have not proven to detect vapors very selectively or at concentrations below a few parts per million. U.S. Pat. No. 4,636,767 teaches the use of organic semiconductor films of phthalocyanine compounds as an alternative to heated metal oxides.
Films formed by sublimation or evaporation exhibit slow responses to vapor concentration changes. U.S. Pat. No. 4,636,767 and U.S. Statutory Invention Registration No. H477 teach the Langmuir-Blodgett technique to form semiconducting thin films of phthalocyanine compounds with gas sensing properties.
The film material is the critical component of the sensor since it determines the sensitivity and the specificity for a particular vapor. The remainder of the sensor can be designed to fit the requirements of the material. For example, if the vapor sensitive material is a weakly conducting organic semiconductor, then the device is fabricated with a large number of electrode fingers in an interdigitated array which provides a short electrode spacing with a large electrode perimeter. This arrangement facilitates the measurement of weak currents through resistive materials.
Neutral dithiolene transition metal complexes (dithiene complexes) have been demonstrated to be electronic conductors (Rosa, E. J., Schrauzer, G. N., J. Phys. Chem., 1969, Vol. 73, p. 3132-3138). They have been used as Q-switch dyes for lasers (East German Patent No. 210416, 1984; "New Dithiene Complexes for Q-Switching and Mode-Locking Infrared Lasers", J. Appl. Phys., Vol. 46, No. 11, p. 4852-3, 1975; Japanese Patent No. 80/585881), as an infrared absorbing material (Japanese Patent No. 86/80106) and as a component of optical recording media (Japanese Patent No. 84/78341). These references do not suggest that the compounds could be used in chemical detection. Nor do these references suggest that the compounds would form stable compressed films on the Langmuir-Blodgett film balance or that they could be applied in thin films by the Langmuir-Blodgett technique. Dithiolenes do not possess the structural features of the materials typically used in Langmuir-Blodgett studies, i.e., long alkyl chains.
In many analytical environments, a single sensor is not sufficient to discriminate between the analyte of interest and the other components of the sample. Several sensors in conjunction with pattern recognition techniques are necessary. In order for pattern recognition techniques to be effective, the individual sensors must have different response patterns to the ambient vapors. To achieve this result, the materials coated onto each of the individual sensors must be different from one another. For this reason, it is important that as many different materials as possible be developed for gas sensing.